Method for making electrode connections to potassium tantalate-niobate



June 6, 1967 TO PUMPS AND GAS MAN/FOLD "iii,"

D. KAHNG ETAL METHOD FOR MAKING ELECTRODE CONNECTIONS TO POTASSIUMTANTALATE-NIOBATE Filed Dec.

FIG. 2

ETCH SURFACE WHERE ELECTRODE TO BE CONNECTED EXPOSE ETCHED SURFACE TOM/CROWAl E OXYGEN PLASMA DEPOSIT ELECTRODE FIGS D. MHNG INVENTORS .1. R.LIGENZA $.H. WEMPLE A TTORNEV United States Patent 3,323,947 METHUD FORMAKING ELECTRUDE CONNEC- TIQNS T1) POTASSHUM TANTALATE-NIOBATE DawonKahng, Somerviile, Joseph R. Ligenza, Califon, and Stuart H. Wemple,Madison, Ni, assignors to Beli Telephone Laboratories, Incorporated, NewYork, N.Y. a corporation of New York Filed Dec. 17, 1%4, tier. No.41%138 2 Claims. (iii. 117-213) This invention relates to themanufacture of devices utilizing compositions within the potassiumtantalate-niobate system (hereinafter referred to as KTN). Moreparticularly, this invention relates to the introduction of acceptorlevels in a localized region of a KTN crystal.

As is described in copending application Ser. No. 35 3,- 049, filed Mar.19, 1964, now Patent No. 3,290,619 (having a common assignee with thisapplication), compositions included within the KTN system haveproperties which adapt them from various electrical applications, suchas in electro-optic and electro-acoustic devices. These applicationsrequire the establishing of electrostatic fieds within the KTN elementand, consequently, the attachment of electrodes to such element.

However, hitherto, optimum realization of the properties of KTN for suchpurposes has been penalized by the poor uniformity of the electro-staticfields which have been achieved therein.

Accordingly, one object of the present invention is an improvement inthe connections to a KTN crystal element whereby a more uniformelectro-static field may be established therein.

In accordance with the invention, it is found advantageous for achievinga more uniform electro-static field within a KTN element to expose atleast the surface portion where the connection is to be made to anoxygen plasma discharge, advantageously a microwave discharge, beforethe attachment of the electrode. Such exposure acts to make the surfacemore p-type. Such exposure has the effect of introducing acceptor levelsin the regions affected.

Accordingly, when the crystal is p-type, this treatment acts to improvesthe ohmic nature of the contact. When the crystal is n-type,'thistreatment enables the making of a good rectifying connection to the bulkof the crystal.

In each case, it is made possible to achieve a more uniformelectro-static field in the crystal.

Although the invention has particular application to surface treatmentof regions to which electrodes are to be attached, in a broader aspectthe invention relates generally to the introduction of acceptor levelsin a surface region of a KTN crystal independent of the purpose. Forexample, the invention has application simply to the formation in KTNcrystal of junctions between regions of different conductivity type.

The invention will be more fully understood from the following moredetailed description taken in conjunction with the accompanying drawing,in which:

FIG. 1 is an elevated and partly sectional view of typical laboratoryoxidation apparatus suitable for treating the KTN element in an oxygenplasma in accordance with the invention;

FIG. 2 shows a flow chart of the basic steps of a process for formingelectrical connections to a KTN crystal in accordance with theinvention; and

FIG. 3 shows a KTN element to which electrodes have been connected.

With reference now more particularly to FIG. 1, the apparatus showncomprises a quartz discharge tube of 1.3 centimeter inner diameterfitted at op osite ends with a pair of electrodes 11 and 12 separated bycentimeters. Electrode 11 is a hollow cylinder with a sealed 3,323,947Patented June 6, 1967 end to provide a work surface 13. Electrode 11 isattached to a hollow cylinder of quartz 15 which is attached to a hollowcylinder of Pyrex glass 16 via a graded glass seal at 17. The wholeelectrode assembly is attached to one end of the discharge tube by aflange at 18. A gas line 19 is provided into the discharge tube from thevacuum pump (not shown) and to the oxygen gas manifold (not shown). Thehollow inside of electrode 11 provides a well 20 for temperaturemeasurement and/or for a coolant.

Electrode 12 extends out through a hollow cylinder of Pyrex glass 21 andthe cylinder is attached to the discharge tube via a graded glass seal23. Conductor 24 is attached to electrodes 11 and 12 and runs to adirectcurrent source 25. A microwave generator (not shown) suppliesmicrowave energy to the resonant cavity 26 which is coupled to thedischarge tube 10 for exciting a microwave discharge therein.

In general, the discharge tube and the electrodes can be of manymaterials. Quartz is a preferred material for the discharge tube becauseof its ability to withstand the high temperature of gas discharges.Silicon is a useful material for the electrodes because it has a lowsputtering yield and can be readily degassed.

The tube dimensions and the parameters of the microwave assembly arevariable over wide limits. What is important is that the assemblyproduce a sufficiently ionized discharge that will contact the KTNelement 27 on the work surface 13.

The crystal to be treated in the apparatus described typically firstneeds to undergo some preliminary treat ment. In particular, theindividual elements, for example cubes with sides about 3 millimeterslong, usually are cut from a larger crystal. This typically involvescutting with a diamond saw and grinding with alumina grit. The cut tingand grinding operations generally result in a damaged surface layerwhich advantageously should be removed. One technique found convenientfor removing the dam aged surface layer involved immersion in moltenpotassium hydroxide at BSD-400 C. for 10-20 minutes and subsequentwashing in distilled water and drying. A wide variety of techniques arefeasible for this preliminary step, so long as removal of the damagedlayer is effected without adversely affecting the surface or introducingundesirable impurities. A beneficial side effect of this surfacetreatment is the elimination of crystal strains associated with thedamaged surface layer.

After the described surface treatment, the KTN crystal is positioned onthe work surface 13 of the anode element .of the apparatus shown in FIG.1 for exposure to the oxygen discharge.

It has been found convenient to operate the apparatus described with anoxygen pressure of about 0.3 millimeter of mercury initially in thedischarge tube. In practice, it is found convenient first to evacuatethe tube to approximately 10 Torr before admitting the oxygen gas. Insome cases, it is found that the initial discharge has impurities suchas C0, C0 and H 0. If the gas is pumped out and the dischargeestablished a few times, a relatively pure oxygen discharge can beobtained. Typically, a direct-current voltage of about 40 volts wasestablished between the cathode and anode, resulting in a current flowof about 20 milliamperes. The discharge was initiated by applying about300 watts of microwave energy at a frequency of 2450 megacycles. The KTNelement was positioned on the holder so that the surface to be treatedfaced the cathode at the opposite end of the tube. Treatment of each oftwo opposite surfaces of the KTN element for ten minutes apiece wasfound adequate. It was estimated that the KTN element was heated toabout 600 C. by the discharge.

Useful operating conditions were found to be variable over wide rangeswithout seriously effecting the results. The most significant parametercharacterizing a glow discharge is its saturation current density. Thisfixes the number of ions striking the Work surface in a given timeinterval. For the conditions described, the saturation current densitywas estimated to be about 25 milliamperes per square centimetercorresponding to about 200x10 ions incident per square centimeter ofsurface per second. The energy of the oxygen ions incident was about 35electron volts and penetration of about 3-5 Angstrom units wasestimated.

The saturation current density which can be achieved varies with thefrequency of the discharge and, accordingly, the time needed tointroduce a prescribed number of oxygen ions will vary inversely withsuch frequency. In particular, it was found that when the discharge wasestablished by applying energy of four megacycles frequency, times ofthe order of hours were needed to achieve equivalent results. However,frequencies higher than several kilomegacycles do not result in anysignificant shortening of time. Actually, even at 2.45 kilomegacycles,substantially shorter times can be used with some slight sacrifice. Thepower supplied to the discharge need be no more than that necessary toensure that the KTN element is in the region of the discharge.Shortening the length of the tube permits operation at lower powerlevels. The D.-C. voltage supplied also can be varied over wide limits.The voltage applied controls within limits the energy with which theoxygen ions bombard the KTN element and, accordingly, the depth to whichthey penetrate. Too low a voltage results in too low a penetration foroptimum results; too high may result in undesirable disturbance to theKTN element. As previously indicated, the specific treatment describedis estimated to introduce oxygen ions to a depth of between 3 and 5Angstrom units. The oxygen pressure which can be employed is alsosubject to variation. However, if too low, there can be insufficient gasto absorb the radiation energy released by ionization and the radiationmay damage the KTN element. If too high, the plasma can become too hot,with possible damage to the KTN element. A range of between .1millimeter of Hg to about 5 millimeters of Hg is preferred. Moreover,instead of a closed discharge tube, it is feasible to fiow the oxygenthrough a discharge region wherein the KTN crystal is supported.

The reason for the improvement effected by exposure to the oxygendischarge as described is not completely understood but appears toresult from the filling of vacancies near the surface with oxygen ions.

After completion of the surface treatment described, there remains toattach the electrodes to the treated faces of the KTN element. This canbe done in the same manner as disclosed in aforementioned applicationSer. No. 353,049, now the said Patent No. 3,290,619, or in any otherfashion suitable to the desired electrode. Suitable electrodes can be ofgold, aluminum and indium-gallium deposited by evaporation.Additionally, nickel electrodes have been deposited by the knownelectroless plating technique.

FIG. 2 shows in a flow chart the basic steps involved and FIG. 3 shows aKTN crystal 30 with a pair of electrodes 31 and 32 attached to oppositesurfaces.

As is pointed out in the last-mentioned copending application, while theproperties responsible for the device characteristics set forth thereinare seen in a broad range of KTN compositions containing at little asabout 20 percent of either of the end members KTaO and KNbO the presentinvention has applicability to the provision of electrode connections tothe entire range of the system including compositions free of one of thetwo end members. Moreover, the principles are applicable even though theKTN element may include trace amounts of impurities either deliberatelyadded or inherently present as a result of the single crystal growthprocess.

What is claimed is:

1. The method of preparing the surface of a crystal whose composition iswithin the potassium tantalateniobate system comprising subjecting saidsurface to an etching process to remove damaged portions; modifying themolecular structure at and near said surface by mounting the crystal inproximity to an anode within a chamber containing said anode and acathode; evacuating the chamber and establishing an oxygen ambient ofbetween about 0.1 and about 5.0 millimeters of mercury pressure therein;applying a predetermined low directcurrent biasing potential betweensaid anode and cathode; creating an oxygen ion plasma in the crystalarea by applying microwave energy to the chamber at such a location, atsuch a sufficient voltage, and at high enough frequency, as to cause aplasma-forming glow discharge to occur in the chamber such that thesurface of the crystal is in the region of the discharge; and allowingthe plasma to bombard the surface for a predetermined time.

2. The steps prescribed in claim 1 followed by the deposition of a metalelectrode on said surface of the crystal.

References Cited UNITED STATES PATENTS 2,750,541 6/1956 Ohl. 2,787,5644/1957 Shockley. 2,883,544 4/1959 Robinson. 2,902,583 9/1959Steingerwald. 3,146,514 9/1964 Knau 2925.3 3,206,336 9/1965 Hora 148-153,212,939 10/1965 Davis 1481.5

WILLIAM I. BROOKS, Primary Examiner.

JOHN F. CAMPBELL, Examiner.

1. THE METHOD OF PREPARING THE SURFACE OF A CRYSTAL WHOSE COMPOSITION ISWITHIN THE POTASSIUM TANTALATENIOBATE SYSTEM COMPRISING SUBJECTING ANDSURFACE TO AN ETCHING PROCESS TO REMOVE DAMAGED PORTIONS; MODIFYING THEMOLECULAR STRUCTURE AT AND NEAR SAID SURFACE BY MOUNTING THE CRYSTAL INPROXIMITY TO AN ANODE WITHIN A CHAMBER CONTAINING SAID AMODE AND ACATHODE; EVACUATING THE CHAMBER AND ESTABLISHING AN OXYGEN AMBIENT OFBETWEEN ABOUT 0.1 AND ABOUT 5.0 MILLIMETERS OF MERCURY PRESSURE THEREIN;APPLYING A PREDETERMINED LOW DIRECTCURRENT BIASING BETWEEN SAID ANODEAND CATHODE; CREATING AN OXYGEN ION PLASMS IN THE CRYSTAL AREA BYAPPLYING MICROWAVE ENERGY TO THE CHAMBER AT SUCH A LOCATION, AT SUCH ASUFFICIENT VOLTAGE, AND AT HIGH ENOUGH FREQUENCY, AS TO CAUSE APLASMA-FORMING GLOW DISCHARGE TO OCCUR IN THE CHAMBER SUCH THAT THESURFACE OF THE CRYSTAL IS IN THE REGION OF THE DISCHARGE; AND ALLOWINGTHE PLASMA TO BOMBARD THE SURFACE FOR A PREDETERMINED TIME.